1. Field of the Invention
The invention relates generally to well logging using resistivity logging tools. More particularly, the invention relates to methods for processing signals from an electro-magnetic well-logging tool to determine formation properties and a desired well path through the formation.
2. Background Art
Wells are generally drilled through formations in the Earth's crust so that materials trapped reservoirs in the formations, such as petroleum, can be recovered. Often a well is drilled horizontally through a reservoir to increase the drainage area in the reservoir. Because many of these formations are layered, the position of the well with respect to the boundaries of the layers can be very important. For example, a poorly placed well may have a much lower productivity than if the well had been drilled in a better location.
Because knowing the exact location of a horizontal bore-hole in a reservoir is critical to maximize production, various techniques have been developed to determine the location of a wellbore in a formation. These techniques include seismic surveys, resistivity measurements, etc. Seismic surveys are the most commonly used in locating the well path in the formation, due to its ability to probe far into the formation. However, seismic surveys are incapable of providing the desired resolution and accuracy to accurately predict the location of the borehole.
Other measurements, such as gamma ray and resistivity measurements, are more accurate and can provide better resolution. Resistivity measurements are often made with electromagnetic (EM) tools. EM logging tools have an elongated support equipped with antennas that are operable as sources or sensors. The antennas on these tools generally comprise loops or coils of conductive wire. In operation, a transmitter antenna is energized by an alternating current to emit EM energy (magnetic field) through the borehole fluid (“mud”) and into the surrounding formations. The magnetic field induces eddy currents in the formation that in turns induce secondary magnetic fields. The secondary magnetic fields then induce signals in the receivers that are disposed at a distance from the transmitter. The magnitudes of the detected signals reflect the formation resistivity. The signals detected by the receiver may be separated into real signals (R-signals) that are in-phase with the transmitter signal and quadrature signals (X-signals) that are out-of-phase with respect to the transmitter signals. By processing the detected signals, a log or profile of the formation and/or borehole properties may be determined. Conventional resistivity tools, however, often cannot “read” far enough into the formation to define the location of the borehole with respect to formation layer boundaries.
Other EM logging tools use propagation techniques to measure the resistivity of the formation. A propagation tool measures the amplitudes, phase shifts, and attenuation of EM signals in the formation to determine the resistivity of the formation.
Recently, a resistivity tool capable of deep reading is disclosed in U.S. Pat. No. 6,188,222 B1 issued to Seydoux et al. and assigned to the assignee of the present invention. This tool includes a long spacing between the transmitter and the receiver, and it takes advantage of the telemetry signals carrying measurement data from down-hole sensors through the formation to a measurement-while-drilling (“MWD”) receiver located higher in the bottom hole assembly (“BHA”). The transmitted signals are decoded by the MWD receiver to extract signal amplitudes, that is used to determine formation resistivity. In addition, changes in the signal amplitude are also used to indicate formation boundaries for determining well location during directional drilling.
Measurement data from logging tools, such as the deep reading resistivity tool disclosed in the Seydoux patent, are typically processed using an inversion method to determine a position of a wellbore with respect to layer boundaries in earth formations. Exemplary inversion methods for calculating a distance between a well logging instrument and a formation boundary from the logging data, for example, may be found in U.K. published patent application GB 2 301 902 A filed by Meyer and in U.S. Pat. No. 6,594,584 B1 issued to Omeragic et al. and assigned to the assignee of the present invention.
An inversion technique, such as that disclosed in the Omeragic patent, involves making an initial estimate or model of the geometry of earth formations and the properties of the formations surrounding the well logging instrument. The initial model parameters are derived in various ways known in the art. An expected logging instrument response is calculated based on the initial model. The calculated response is then compared with the measured response of the logging instrument. The difference between the calculated response and the measured response is used to adjust the parameters of the initial model. The adjusted model is again used to calculate an expected response of the well logging instrument. The expected response for the adjusted model is compared to the measured instrument response, and any difference between them is used to adjust the model. This process is repeated until the differences between the expected response and the measured response fall below a pre-selected threshold.
While these prior art resistivity tools and methods can sometimes provide satisfactory results in locating well-bore in the formations, better apparatus and methods are needed, in particular for geosteering during drilling horizontal wells.